Various types of fluid flow meters have been proposed for detecting and measuring characteristics relating to fluid and/or fluid flow. For instance, thermal type air flow meters have been proposed that measure air flow rate utilizing a resistance temperature characteristic of a hot wire. These thermal type air flow meters have been proposed for measuring a flow rate of air taken into an internal combustion engine.
The thermal type air flow meter described in Japanese Patent No. 9-287985A, for example, has a sensor body which is inserted in an air intake pipe of an internal combustion engine. A bypass passage is formed inside the sensor body through which part of the air flowing through the intake pipe passes. The air flow meter measures a flow rate of the air flowing through the intake pipe (i.e. an intake air flow rate) based on the amount of heat radiated from a heater element installed in the bypass passage. Such a prior-art thermal type air flow meter can make stable measurement with little variation when measuring a steady air flow.
However, prior art fluid flow detecting devices suffer from certain disadvantages. For instance, in prior art thermal-type air flow meters, measurement accuracy decreases when measuring a pulsating flow, which results from intake pulsation of an internal combustion engine, and which reduces the velocity of the air flowing through the bypass passage.
More specifically, as shown in FIG. 5, in a steady air flow air flow separation occurs on both sides of a sensor body 100, so that the velocity of the air exiting the bypass passage does not decrease substantially. In other words, the loss caused when the air flow exiting the bypass passage joins the main flow is relatively small, so that the velocity of the air flowing through the bypass passage is not reduced substantially. This enables stable air flow measurement.
However, as shown in FIG. 5, in a pulsating flow the air flow separation occurring on both sides of the sensor body 100 is relatively small so that the main flow velocity near the exit of the bypass passage remains relatively high. This causes the air flow exiting the bypass passage to suffer a large loss when joining the main flow. As a result, the velocity of the air flowing through the bypass passage decreases substantially. This condition can cause measurement errors. Therefore, there remains a need for a more accurate fluid flow detecting apparatus.